The lateral sway of a multi-storey steel frame should be limited so as to ensure the comfort of the occupants and for the protection of mechanical and architectural systems. This paper investigates the applicability of Schueller\'s equation for predicting sway of rigid steel frames, and proposes a number of modifications to the equation so that it can produce results that are almost identical to those given by accurate Finite Element (FE) analysis. The behaviour of irregular steel frames has also been studied and proposals are made so that Schueller\'s Equation can also be used to predict sway of such frames.